In hydraulic-elastomeric mounts such as used for mounting an internal combustion engine in a motor vehicle, a hydraulic damping decoupler can be employed to eliminate undesirable hydraulic damping at certain small vibratory amplitudes and low frequencies and thereby better isolate the vibrations of the engine under these conditions. Such decouplers are normally reciprocating devices which are mounted on a partition separating the two orifice connected chambers of the mount and operate to effect alternating volume change in the chambers so as to not force flow through the orifice and thereby eliminate damping in this mode. Examples of such mounts are disclosed in detail in U.S. Pat. Nos. 4,588,173 and 4,765,601 assigned to the assignee of this invention.
While the performance of such decouplers has proven generally satisfactory, it was found that they typically slap against the partition surfaces determining the limits of their movement and that such slaps can produce a discernible noise that is objectionable. However, it was further found that this noise problem could be solved by constructing the decoupler with faces that cause the decoupler to rock rather than slap and by additionally allowing some bypass flow around the decoupler to smooth the transition between damping and no damping (decoupling); that is, reduce the abruptness of decoupling. These solutions are disclosed in detail in U.S. Pat. Nos. 4,679,777 and 4,664,363, respectively, assigned to the assignee of this invention.
While these solutions to the noise problem have proven satisfactory, the manner of constructing the bypass around the decoupler has been found to be critical to producing consistent results. For example, in the above U.S. Pat. No. 4,664,363, the bypass is constructed as a passage between the decoupler and the side of the partition opening in which it is mounted with a baffle and reservoir then further added to the partition to assure the maintenance of some damping and smooth decoupler operation. But because of manufacturing tolerances, it is difficult to maintain all this structure in precise relationship to obtain the exact results desired. Furthermore, this bypass construction involves considerable redesign work where it is desired to change the spring rate and/or damping characteristics for a different application of the mount.